As an automatic transmission that achieves seven forward speeds (forward speed-ratios) by using three planetary gear sets, for example, techniques disclosed in Patent Document 1 and Patent Document 2 are known. Patent Document 1 discloses an automatic transmission which achieves seven forward speeds by using six friction elements and three single-pinion-type planetary gear sets. The single-pinion-type planetary gear set has an advantage in transfer efficiency, gear noise and also durability, and does not need to reduce a diameter of pinion gear. Moreover, in the similar manner, Patent Document 2 discloses an automatic transmission which achieves six forward speeds to eight forward speeds by using five friction elements and three single-pinion-type planetary gear sets.
Patent Document 1: Japanese Patent Application Publication No. 2004-176765
Patent Document 2: U.S. Pat. No. 6,514,170, FIG. 11a, FIG. 11b